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| Maximising Torsional Stiffness |
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| awemawson:
I've bitten the bullet and ordered the steel to re-make the attachment of my hedge flail that I've been banging on about in various other posts, and as I start to build it I will start a suitable thread - however this is more a theoretical question. Imagine a ladder like structure - two parallel rails joined by 'rungs' for half the length, but the other half only joined at the extreme end. This end can (hopefully) be regarded as firmly fixed. The end where the 'rungs' are has a very uneven load creating a significant twist or torque. Am I correct in thinking there is virtually nothing to be gained by adding rungs to increase the stiffness to resist this twist, and about the only way is to add a third member parallel but above (or below), and triangulate the structure ? Conceptually imagine two rails, built into a wall, projecting horizontally out from the wall, joined together for the outer half of their length, then a force trying to twist them along their length. (The 'rungs' are the main body of the flail and the extreme ends will be joined to the tractor axle. The flail head and arm weighs perhaps 75-100 kgs and project only to one side) |
| vtsteam:
The longer the span the easier the twist. I'm assuming a welded structure. And pin ends at the wall instead of fixed. So I think more rungs will help some. But not as much as substantially increasing thickness of the structure (ie. your diagonals out of the plane of the ladder.) This is very general talk, on my part. |
| Pete W.:
Hi there, Andrew, Make the stiles high and add rungs both top and bottom AND herringbone strutting. (Analogy - floor joists.) |
| PekkaNF:
Not very sure if I follow it completely, but if I got the question right: * Plate (thin but wide) is very stiff on one direction and very floppy on other directions. * Perfectly round tube has same torsional restistance on every point of it's circumsfere, I.E. same stiffnes in every direction. * Closed profile tends to be more stiff to twisting than open. Compare pipe and C. * Generlly more you have dimenssion of the profile in one direction, more stiffness you have for weight....until the structure collapses when a rock, dent ot angly sparrow hits it and it collapses. You see aluminium pipe bike bodies, If only strength were only consideration, beer can thikness would be pretty close, but it would collapse when something touches it. You are right on geodesic ladder format, ancient aeroplane fuselages are good eaxample and modern radio masts. Booms on mobile devices tend to be hight strengt square steel structure that properties are optimized and their edge is reinfoced. Round shape would be great, but terminating ito an actuator or pivot is not nice. Square tube is a very good compromize. I could not find any paper...but I'm not mechanial engineer, I see engineers calculating and dimenssioning this kind of structures (different steel and different product) all the time. This is example of steel/section type that is used. I have seen some modern ones and wall thiknesses look pretty thin. Joints and hard points have cast or welded reinfosments of rather big dimenssions. http://www.ruukki.us/~/media/Files/Steel-products/US%20Tubular%20products%20-%20Brochures/Ruukki-Optim-High-strength-hollow-sections.pdf http://www.ruukki.us/References/Lifting/Time-Manufacturing---Aerial-lifts-that-lift-more-and-reach-higher http://www.ruukki.us/Products-and-solutions/Steel-products/First-class-steel-products-and-services-to-a-wide-range-of-customer-industries/Lifting Not sure if I'm answering the question at all? Pekka |
| vtsteam:
There are traditional engineering formulas for calculating deflection in trusses to give specific answer to a problem. But to try to speak on the level of intuitive understanding of what happens with a mental picture, a simple ladder with two people at both ends can be twisted to some degree with reasonable strength. If you remove several of the rungs toward one end, the holders will be able to twist it further using the same force. The two stiles will deflect more in the area where the rungs have been removed. They will deflect oppositely, and attempt to form a helix in that section. The degree of deflection is the part that is important to Andrew, but that is not possible say without doing the formulas, or relying on experience of a similar setup and set of forces. I do think, also just intuitively, that the end with the single rung should have triangular corner gusset plates for longer life since fatigue from flexing is going to be a consideration at those welds. |
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